Synchronous machine having control coils for compensating mechanical oscillations of the rotor

ABSTRACT

In order to compensate for the effects of mechanical oscillations of the salient pole rotor in a synchronous machine in a simple manner, control coils are provided which generate a controllable quadrature-axis magnetic field. The control coils are arranged on the salient pole rotor in such a way that the quadrature-axis magnetic field generated by the control coils is superimposed in the air gap on the controllable direct-axis magnetic field generated by exciting coils enclosing each pole body of the salient pole rotor.

BACKGROUND OF THE INVENTION

The present invention relates to a synchronous machine having a salientpole rotor.

A method and an arrangement for reducing at least one frequencycomponent of a periodic pulsation is described in European Patent No.EP-A1-0 268 160. In this arrangement, a synchronous generator is drivenby a drive motor. Particularly a high-power synchronous generator drivenby a diesel engine, and pulsations of an electrical quantity arecompensated at the output of the synchronous generator with a frequencythat deviates from the synchronous generator's own frequency. Thecontrol is effected on the path formed by the diesel engine, generatorand power system by intervening in the excitation of the generator(i.e., by controlling the direct-axis magnetic field) or by influencingthe operating conditions of the diesel engine (quantity and timing ofthe fuel injection are controlled), or by an impedance (adjustableadditional load) arranged at the output of the generator. These types ofcontrol are costly, since in some circumstances one of these measures isnot sufficient by itself to achieve a complete compensation of theperiodic pulsation.

An object of the present invention is to provide a synchronous machineof the type mentioned above, in which the effects of mechanicaloscillations of the salient pole rotor, which can be attributed toinfluences both on the drive side and on the load side, are compensatedin a simple manner.

SUMMARY OF THE INVENTION

In the synchronous machine of the present invention, in addition to theexciting coils which generate a controllable direct-axis magnetic fieldin a conventional manner, control coils are provided which generate acontrollable quadrature-axis magnetic field. The control coils arearranged on the salient pole rotor in such a way that thequadrature-axis magnetic field generated by the control coils issuperimposed on the direct-axis magnetic field in the air gap.Therefore, an additional intervention parameter is obtained forcompensating the fluctuations caused by mechanical oscillations of thesalient pole rotor on the electrical side.

As a result of the control of the quadrature-axis magnetic fieldsimultaneously with the mechanical oscillations, a regularly rotatingfundamental flux resulting from the direct-axis magnetic field andquadrature-axis magnetic field can be accomplished. Comparable operatingconditions to those of a regularly rotating salient pole rotor are thusobtained, that is to say without oscillations.

The arrangement of these control coils is advantageous in high-powerslow-running synchronous generators which are driven, for example, bydiesel engines having an irregular torque. Until this time, generatorsof this type have had to be dimensioned in such a way that they have acorrespondingly high mass moment of inertia, which is unfavorable withrespect to costs and to weight and space requirements.

The quadrature-axis magnetic field generated by the control coils issuperimposed on the direct-axis magnetic field. In order to achievethis, it is possible, to provide control coils which are arranged onadditional poles designed in the manner of commutating poles. Apart fromthe constructional problems and the manufacturing outlay for suchadditional poles, with such an arrangement of the control coils, thepole pitch for the direct-axis magnetic field and hence the machineutilization (output for a given machine volume) would be reduced to arelatively high degree. Moreover, with the aforementioned arrangement itis not readily possible to achieve sufficiently high amplitudes for thequadrature-axis magnetic field.

In an embodiment of the synchronous machine of the invention, both ahigh pole pitch for the direct-axis magnetic field and a sufficientlyhigh amplitude for the quadrature-axis magnetic field is ensured. Byvirtue of the arrangement of the control coils according to theinvention, the quadrature-axis magnetic field essentially permeates onlythe pole shoes, that is to say only the upper pole region adjoining theair gap. The pole core and the yoke, on the other hand, carry only thedirect-axis magnetic flux interlinked with the exciting coils. Such anarrangement experiences relatively rapid changes in the quadrature-axismagnetic field and hence rapid compensation of interfering oscillationsof the salient pole rotor.

In a further embodiment of the present invention, at least one damperframe is arranged for each pole to compensate for dynamic changes ofstate as can occur, for example, with loading and unloading. The damperframes can be arranged symmetrically with the grooves holding thecontrol coils.

In contrast to a conventional damper cage, these damper frames do notact on the quadrature-axis magnetic field. Accordingly, they also do notinfluence the compensation of the mechanical oscillations of the salientpole rotor effected by the quadrature-axis magnetic field. As a resultof its effect on the quadrature-axis magnetic field, a damper cageshould be employed in the synchronous machine of the invention only ifthe required control times for the quadrature-axis magnetic field aregreater by at least one power of ten than the determinative subtransienttime constants of the damper cage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section of an unwound salient pole rotor of thesynchronous machine according to the invention.

FIG. 2 is a cross section through the salient pole rotor of FIG. 1 alongthe line II--II,

FIG. 3 is a line graph showing the direct-axis magnetic field and thequadrature-axis magnetic field in an idealized representation,

FIG. 4 is a magnetomotive force diagram of the synchronous machineaccording to the invention, and

FIG. 5 is a pole shoe of the salient pole rotor according to FIG. 2 withdamper frame and without coils (section II--II as in FIG. 2).

DETAILED DESCRIPTION

In FIG. 1, reference numeral 1 denotes a salient pole rotor of asynchronous machine having poles 2 which comprise a pole body 3 and apole shoe 4 placed thereon. Each pole shoe 4 has a groove 5 in itscenter which extends in the longitudinal axis of the salient polerotor 1. Also, recesses 6 are provided on both longitudinal sides ofeach pole body 3 which extend parallel to the groove 5 in each case.Each pole body 3 of the salient pole rotor 1 is enclosed by an excitingcoil 7 (see FIG. 2). The longitudinal sides of the exciting coils 7 arelocated in the two recesses 6 of a pole body 3.

In addition to these conventional exciting coils 7, the salient polerotor 1 has control coils 8. In each case two longitudinal sides of theadjacent control coils 8 are located together in the groove 5 of thepole shoes 4. In each case, a control coil 8 encloses the adjacent poleshoe halves of two poles 2 (see FIG. 2).

The control coils 8 are fastened with, a stopper key in the grooves 5,bands, and/or supporting rings or supporting caps in the front-endregion of the rotor.

In FIG. 1 the current direction flowing into the plane of the drawing isindicated by a cross. The current direction flowing out of the plane ofthe drawing is indicated by a dot. In FIG. 2 the direction of thecurrent I is indicated by arrows. With the current directionsillustrated in FIGS. 1 and 2, the magnetic flux curves of the salientpole rotor 1 represented in FIG. 1 are obtained. The exciting coils 7generate the direct-axis magnetic field B_(o) (solid arrows), while thecontrol coils 8 generate the quadrature-axis magnetic field denoted byΔB (dashed arrows).

For the sake of simplicity in the following qualitative analysis, it isassumed that the direct-axis field B_(o) generated by the exciting coils7 has a rectangular waveform. Corresponding to this direct-axis fieldB_(o) is the direct-axis field fundamental amplitude (shown in dashedlines in FIG. 3) B_(d) ¹ =B_(o)π⁴ cosα, where α denotes the electricalangle of the pole tip (angular distance from the middle of the pole gapto the edge of the pole shoe 4).

The quadrature-axis magnetic field ΔB generated by the control coils 8corresponds to a quadrature-axis field fundamental amplitude of B_(q) ¹=ΔB.sub.π ⁴ (1-sinα). As a result of the quadrature-axis field ΔBsuperimposed on the direct-axis field B_(o), the magnetic flux densityis reduced in one pole half by ΔB (in each case the left pole shoe halfin FIG. 1) and increased in the other half by ΔB (in each case the rightpole shoe half in FIG. 1).

Given a pole-pitch factor of approximately 0.7, which is customary inpractice (the corresponding electrical pole tip angle α is thenapproximately 27°), B_(q) ¹ /B_(d) ¹ ≈0.6 ΔB/B_(o) is obtained for thequotient of interest from quadrature-axis field and direct-axis fieldfundamental amplitude. For a quadrature-axis field that is 10% of thedirect-axis field, that is to say B_(q) ¹ =0.1 B_(d) ¹, a flux densitydeviation of ΔB/B_(o) ≈±0.17 is required. This achieves a displacementof the flux vector by the angle ± arctan 0.1≈±5.7°.

Given an unsaturated synchronous machine the quadrature-axis componentof magnetomotive force θ₂ of the control coil 8 required for a fluxdensity deviation ΔB/B_(o) ≈±0.17 is thus approximately 17% of themagnetomotive force difference θ.sub.μ, which corresponds to the vectorsum of the direct-axis component of magnetomotive force θ₁ and thearmature reaction θ_(A) (see FIG. 4). Given a saturated magneticcircuit, the flux density on one pole shoe half decreases more than itincreases at the same time on the other pole shoe half. For theresulting quadrature-field component of 10% described above, a slightlyhigher quadrature-axis component of magnetomotive force θ₂ of thecontrol coil 8 is expected of, for example, around 20% of themagnetomotive force difference θ.sub.μ. In comparison to the overalldirect-axis component of magnetomotive force θ₁ of the exciting coil 7,the quadrature-axis component of magnetomotive force θ₂ of the controlcoil 8 required for the quadrature-axis field is relatively small--seeFIG. 4. The principal component of the direct-axis component ofmagnetomotive force θ₁ is required for compensating the armaturereaction θ_(A) caused by the current-carrying stator winding.

As FIG. 1 shows, the quadrature-axis field ΔB generated by the controlcoils 8 permeates essentially only the pole shoes 4 on the rotor side.The pole bodies 3 on the other hand carry only the direct-axis fieldB_(o) interlinked with the exciting coils 7. An arrangement of this typeexperiences relatively rapid changes in the quadrature-axis field.

With regard to the electromagnetic compensation processes in the case ofdynamic changes of state, for example in the case of load surges, adamper cage should only be provided if the required control time for thequadrature-axis field ΔB is greater by at least a power of ten than thedeterminative subtransient time constants of the compensation processesin the synchronous machine. Given conventional machine dimensions, thesesubtransient time constants are in the order of magnitude of 10 to 50msec.

If the required control time is of the same order of magnitude (as thesubtransient time constant) however, then at least one damper frame 9can be advantageously provided on each pole shoe 4 in the synchronousmotor according to the invention for damping compensation processes.Such a damper frame 9 acts only on the direct-axis magnetic field B_(o)and hence does not influence the control behavior for thequadrature-axis magnetic field ΔB. According to FIG. 5 the damper frames9 are arranged symmetrically with the grooves 5. The damper frames 9 areexpediently attached to the pole shoes 4 before insertion of the controlcoils 8.

The constructional design of the invention can be varied from thoseshown in the drawing in a variety of ways. For example, instead of onegroove 5 in the center of the pole in each case, a plurality of groovesmay be present to accommodate a control winding that is correspondinglyspatially distributed. The damper frames 9 are expediently spatiallyplaced at the front ends of the poles in such a way that they arealready completed in the unwound state and permit a subsequent insertionof the windings 7 and 8.

In addition to control as a function of the mechanical oscillations themagnetic field is controlled as a function of the mechanicaloscillations of the salient pole rotor to achieve a regularly rotatingfundamental flux resulting direct-axis magnetic field andquadrature-axis magnetic field, the quadrature-axis field controlaccording to the invention can also be advantageously employed for othercontrol tasks such as, for example, supply-side active load oscillationsor wind power stations.

We claim:
 1. A synchronous machine, comprising:a salient pole rotorincluding a plurality of individual poles, each pole comprising a polebody, and a pole shoe, each pole shoe comprising first and secondhalves; a plurality of exciting coils, each of said exciting coilsencircling one of said individual poles in said salient pole rotor, eachof said exciting coils capable of conducting electric current andgenerating a controllable direct-axis magnetic field, and a plurality ofcontrol coils, each of said control coils coupled within poles shoes ofadjacent individual poles in said salient pole rotor, each of saidcontrol coils capable of conducting electric current and generating acontrollable quadrature-axis magnetic field, quadrature-axis anddirect-axis magnetic fields flow in equivalent directions over the firsthalf of each of said pole shoes and in opposite directions over thesecond half of each of said pole shoes.
 2. The synchronous machine ofclaim 1, wherein said salient pole rotor is capable of being rotatedabout an axis, and each of said pole shoes in the poles of said salientpole rotor comprises:a groove extending parallel to the axis of saidsalient pole rotor, said groove capable of receiving said control coilssuch that each of said control coils is coupled within grooves ofadjacent poles in said salient pole rotor.
 3. The synchronous machine ofclaim 2, wherein each of said grooves are centered within said poleshoes.
 4. The synchronous machine of claim 1, wherein each of the polesof said salient pole rotor comprises:at least one damper frame coupledto said pole shoe such that said damper frame suppresses pulsatingchanges in magnitude only in said direct-axis magnetic field.
 5. Thesynchronous machine of claim 2, wherein each of the poles of saidsalient pole rotor comprises:at least one damper frame coupled to saidpole shoe such that said damper frame suppresses pulsating changes inmagnitude only in said direct-axis magnetic field.
 6. The synchronousmachine of claim 3, wherein each of the poles of said salient pole rotorcomprises:at least one damper frame coupled to said pole shoe such thatsaid damper frame suppresses pulsating changes in magnitude only in saiddirect-axis magnetic field.
 7. The synchronous machine of claim 5,wherein said damper frame is coupled symmetrically to said groove ofsaid pole shoe.
 8. The synchronous machine of claim 6, wherein saiddamper frame is coupled symmetrically to said groove of said pole shoe.9. The synchronous machine of claim 1, wherein said quadrature-axismagnetic field is capable of being controlled such that an amplitude ofsaid quadrature-axis magnetic field is a function of mechanicaloscillations of the salient pole rotor, where combining saidquadrature-axis and direct-axis magnetic fields results in a regularlyrotating magnetic flux in said synchronous machine.
 10. The synchronousmachine of claim 2, wherein said quadrature-axis magnetic field iscapable of being controlled such that an amplitude of saidquadrature-axis magnetic field is a function of mechanical oscillationsof the salient pole rotor, where combining said quadrature-axis anddirect-axis magnetic fields results in a regularly rotating magneticflux in said synchronous machine.
 11. The synchronous machine of claim3, wherein said quadrature-axis magnetic field is capable of beingcontrolled such that an amplitude of said quadrature-axis magnetic fieldis a function of mechanical oscillations of the salient pole rotor,where combining said quadrature-axis and direct-axis magnetic fieldsresults in a regularly rotating magnetic flux in said synchronousmachine.
 12. The synchronous machine of claim 4, wherein saidquadrature-axis magnetic field is capable of being controlled such thatan amplitude of said quadrature-axis magnetic field is a unction ofmechanical oscillations of the salient pole rotor, where combining saidquadrature-axis and direct-axis magnetic fields results in a regularlyrotating magnetic flux in said synchronous machine.
 13. The synchronousmachine of claim 5, wherein said quadrature-axis magnetic field iscapable of being controlled such that an amplitude of saidquadrature-axis magnetic field is a function of mechanical oscillationsof the salient pole rotor, where combining said quadrature-axis anddirect-axis magnetic fields results in a regularly rotating magneticflux in said synchronous machine.
 14. The synchronous machine of claim6, wherein said quadrature-axis magnetic field is capable of beingcontrolled such that an amplitude of said quadrature-axis magnetic fieldis a function of mechanical oscillations of the salient pole rotor,where combining said quadrature-axis and direct-axis magnetic fieldsresults in a regularly rotating magnetic flux in said synchronousmachine.
 15. The synchronous machine of claim 7, wherein saidquadrature-axis magnetic field is capable of being controlled such thatan amplitude of said quadrature-axis magnetic field is a function ofmechanical oscillations of the salient pole rotor, where combining saidquadrature-axis and direct-axis magnetic fields results in a regularlyrotating magnetic flux in said synchronous machine.
 16. The synchronousmachine of claim 8, wherein said quadrature-axis magnetic field iscapable of being controlled such that an amplitude of saidquadrature-axis magnetic field is a function of mechanical oscillationsof the salient pole rotor, where combining said quadrature-axis anddirect-axis magnetic fields results in a regularly rotating magneticflux in said synchronous machine.